The separation of gas mixtures containing hydrogen and light hydrocarbons is an important and widely-used operation in the refining and petrochemical industries. Many of these gas mixtures contain hydrogen, methane, major amounts of ethane and propane, and lower amounts of heavier saturated hydrocarbons. The recovery of hydrogen from such gas mixtures is an economically important operation in the refining industry. Other gas mixtures, for example gas mixtures produced by steam pyrolysis of saturated hydrocarbons, contain hydrogen, methane, and unsaturated hydrocarbons including ethylene and propylene. The recovery of ethylene and propylene from these mixtures is a large and economically important segment of the petrochemical industry. It is desirable in many cases to recover product quality hydrogen along with the main ethylene and propylene products. The recovery of methane-rich fuel gas also may be desirable.
The separation of these gas mixtures is usually accomplished by cryogenic condensation and fractionation methods, which require large amounts of refrigeration at low temperatures. Many methods have been proposed to provide this refrigeration for the recovery of C2 or C3 and heavier hydrocarbons in combination with an upgraded hydrogen product stream. These methods include work expansion of the upgraded hydrogen product gas, refrigeration systems using mixed refrigerants, conventional vapor compression refrigeration systems, Joule-Thomson expansion refrigeration, and various combinations of these refrigeration systems. Other processes utilize absorption for the recovery of C2 or C3 and heavier hydrocarbons and for the removal of light hydrocarbon impurities from the hydrogen product stream.
U.S. Pat. No. 5,979,177 describes a process utilizing a binary mixed refrigerant refrigeration system to recover ethylene and hydrogen from cracked gas in an ethylene plant. U.S. Pat. No. 5,626,034 describes a process utilizing two mixed refrigerant refrigeration systems to recover ethylene and hydrogen from cracked gas. However, most ethylene plants utilize cascaded vapor compression-type ethylene and propylene refrigeration systems supplemented with fuel gas expanders to recover ethylene and hydrogen as described in U.S. Pat. Nos. 5,452,581, 5,421,167 and 4,629,484.
A cold absorption process is described in U.S. Pat. No. 5,414,168 which utilizes an internally generated hydrocarbon stream as a solvent with work expansion of the upgraded hydrogen product gas to provide refrigeration for recovery of olefinic hydrocarbons and purified hydrogen from a catalytic dehydrogenation unit effluent gas stream. Another cold absorption process is disclosed in U.S. Pat. No. 5,333,462 which utilizes Joule-Thomson expansion of the separated hydrocarbon liquids to provide refrigeration for recovering heavy hydrocarbons and hydrogen from catalytic cracking off-gas and an auxiliary gas, which is partially condensed to provide the absorption solvent.
U.S. Pat. No. 4,256,476 discloses a process utilizing only Joule-Thomson expansion of the separated hydrocarbon liquids to recover ethane and hydrogen from thermal hydrocracking off-gases. U.S. Pat. No. 4,749,393 describes a cryogenic process utilizing work expansion of the upgraded hydrogen product gas and Joule-Thomson expansion of the separated hydrocarbon liquids to provide refrigeration for recovery of heavy hydrocarbons and hydrogen from hydrogen-lean feed gases. U.S. Pat. No. 4,559,069 describes a multistage fractional condensation process utilizing Joule-Thomson expansion of the separated hydrocarbon liquids and auxiliary vapor compression-type C2 and C3 refrigeration units to recover hydrogen and heavy hydrocarbons from multiple feed streams.
U.S. Pat. No. 6,266,977 describes a process to recover C2 or C3 and heavier hydrocarbons, including ethylene and/or propylene, utilizing a closed-loop gas expander refrigeration system but does not address the recovery of an upgraded hydrogen product stream or a methane-rich product stream.
Gas expander refrigeration systems of the open-loop and closed-loop type, including some which use nitrogen as the refrigerant, are described for use in hydrocarbon gas liquefaction processes in U.S. Pat. Nos. 6,041,620, 6,041,621, and 6,308,531; PCT Applications WO 95/27179 and WO 97/13109; and German Patent 24 40 215.
There is a need in the refining and petrochemical industries for improved refrigeration methods for the recovery of C2 or C3 hydrocarbons in combination with the recovery of hydrogen, particularly at warmer temperature levels of xe2x88x9250xc2x0 F. to xe2x88x92300xc2x0 F. The present invention, as described below and defined by the claims which follow, addresses this need with several closed-loop gas expander refrigeration systems for recovering C2 or C3 hydrocarbons, hydrogen, and optionally methane from hydrogen-hydrocarbon mixtures.
The invention relates to a method for the recovery of hydrogen and one or more hydrocarbons having one or more carbon atoms from a feed gas containing hydrogen and the one or more hydrocarbons, which process comprises (a) cooling and partially condensing the feed gas to provide a partially condensed feed; (b) separating the partially condensed feed to provide a first liquid stream enriched in the one or more hydrocarbons and a first vapor stream enriched in hydrogen; (c) further cooling and partially condensing the first vapor stream to provide an intermediate two-phase stream; and (d) separating the intermediate two-phase stream to yield a further-enriched hydrogen stream and a hydrogen-depleted residual hydrocarbon stream. Some or all of the cooling in (a), or in (c), or in (a) and (c) is provided by indirect heat exchange with cold gas refrigerant generated in a closed-loop gas expander refrigeration cycle.
The cooling in (a) may be effected in a first heat exchange zone and the further cooling in (c) may be effected in a second heat exchange zone. The method may further comprise introducing the first liquid stream into a stripping column, and withdrawing therefrom a liquid stream further enriched in the one or more hydrocarbons and a residual vapor stream comprising hydrogen and portions of the one or more hydrocarbons.
The method may further comprise reducing the pressure of the hydrogen-depleted residual hydrocarbon stream of (d) to yield a reduced-pressure residual hydrocarbon stream and warming the reduced-pressure residual hydrocarbon stream in the second heat exchange zone by indirect heat exchange with the first vapor stream enriched in hydrogen to provide a portion of the cooling in (c), thereby providing a warmed residual hydrocarbon stream. The method may further comprise combining the residual vapor stream from the stripping column and the warmed residual hydrocarbon stream from the second heat exchange zone to provide a combined residual stream, and warming the combined residual stream by indirect heat exchange with the feed gas in the first heat exchange zone, thereby providing a portion of the cooling of the feed gas in (a).
The cold gas refrigerant generated in the closed-loop gas expander refrigeration cycle may provide cooling in the first and second heat exchange zones by the steps of
(1) compressing and cooling a refrigerant gas to provide a cooled compressed refrigerant gas and dividing the cooled compressed refrigerant gas into a first and a second cooled refrigerant gas stream;
(2) work expanding the first cooled refrigerant gas stream to provided a cooled work-expanded refrigerant gas stream;
(3) further cooling and reducing the pressure of the second cooled refrigerant gas stream to provide a cooled reduced-pressure refrigerant gas stream, wherein reducing the pressure is effected by either work expansion or Joule-Thomson expansion across a throttling valve;
(4) warming the cooled reduced-pressure refrigerant gas stream in the second heat exchange zone to provide at least a portion of the cooling of the first vapor stream in (c), thereby providing a warmed reduced-pressure refrigerant gas stream; and
(5) combining the cooled work-expanded refrigerant gas stream of (2) and the warmed reduced-pressure refrigerant gas stream of (4) to provide a combined reduced-pressure refrigerant gas stream and warming the combined reduced-pressure refrigerant gas stream in the first heat exchange zone to provide at least a portion of the cooling of the feed gas in (a), thereby warming the combined reduced-pressure refrigerant gas stream to provide the refrigerant gas of (1).
The refrigerant gas may be selected from the group consisting of nitrogen, methane, a mixture of nitrogen and methane, and air.
The method may further comprise warming the further-enriched hydrogen stream of (d) in the first and second heat exchange zones to provide by indirect heat exchange a portion of the cooling of the feed gas in (a) and a portion of the cooling of the first vapor stream in (c).
The cooling in (a) and (c) may be effected in a first heat exchange zone and the method may further comprise introducing the first liquid stream of (b) into a distillation column and withdrawing therefrom a liquid stream enriched in hydrocarbons containing two or more carbon atoms and a residual vapor stream enriched in methane. The intermediate two-phase stream of (c) may be introduced into the distillation column.
The method may further comprise warming the residual vapor stream in the first heat exchange zone to provide by indirect heat exchange at least a portion of the cooling of the feed gas in (a). The method also may further comprise cooling and partially condensing the further-enriched hydrogen stream of (d) in a second heat exchange zone to provide an additional intermediate two-phase stream, and separating the additional intermediate two-phase stream to yield a hydrogen product stream and an additional hydrogen-depleted residual hydrocarbon stream. In addition, the hydrogen product stream may be warmed in the first and second heat exchange zones to provide by indirect heat exchange a portion of the cooling of the feed gas in (a) and a portion of the cooling of the further-enriched hydrogen stream.
The method may further comprise reducing the pressure of the additional hydrogen-depleted residual hydrocarbon liquid stream to yield a reduced-pressure residual hydrocarbon liquid stream, warming the reduced-pressure residual hydrocarbon liquid stream in the second heat exchange zone to yield a two-phase residual hydrocarbon liquid stream, separating the two-phase residual hydrocarbon stream to yield a residual hydrocarbon vapor stream and an enriched hydrocarbon liquid stream, and introducing the enriched hydrocarbon liquid stream into the distillation column as reflux. In addition, the residual hydrocarbon vapor stream may be warmed in the first heat exchange zone to provide a portion of the cooling of the feed gas in (a).
A portion of the feed gas stream may be cooled by indirect heat exchange with one or more hydrocarbon-rich liquid streams withdrawn from a lower part of the distillation column to provide a cooled feed stream and one or more vaporized hydrocarbon-rich streams, the one or more vaporized hydrocarbon-rich streams may be returned to the distillation column to provide boil-up therein, and the cooled feed stream may be combined with the partially condensed feed of (a).
The cold gas refrigerant generated in the closed-loop work expander refrigeration cycle may provide cooling in the first and second heat exchange zones by the steps of
(1) providing a compressed refrigerant gas, cooling the compressed refrigerant gas to provide a cooled compressed refrigerant gas, and dividing the cooled compressed refrigerant gas into a first and a second cooled refrigerant gas stream;
(2) work expanding the first cooled refrigerant gas stream to a first pressure to provided a cooled work-expanded refrigerant gas stream;
(3) further cooling and reducing the pressure of the second cooled refrigerant gas stream to a second pressure to provide a cooled reduced-pressure refrigerant gas stream, wherein reducing the pressure is effected by either work expansion or Joule-Thomson expansion across a throttling valve, and the second pressure is lower than the first pressure;
(4) warming the cooled reduced-pressure refrigerant gas stream in the second heat exchange zone to provide at least a portion of the cooling of the further-enriched hydrogen stream of (d), thereby providing a warmed reduced-pressure refrigerant gas stream;
(5) further warming the warmed reduced-pressure refrigerant gas stream in the first heat exchange zone to provide a portion of the cooling of the feed gas in (a), thereby providing a further-warmed reduced-pressure refrigerant gas;
(6) warming the cooled work-expanded refrigerant gas stream of (2) in the first heat exchange zone to provide at least a portion of the cooling of the feed gas in (a), thereby providing a warmed work-expanded refrigerant gas; and
(7) compressing the further-warmed reduced-pressure refrigerant gas of (5) and the warmed work-expanded refrigerant gas of (6) to provide the compressed refrigerant gas in (1).
The refrigerant gas may be selected from the group consisting of nitrogen, methane, a mixture of nitrogen and methane, and air.
In another embodiment, the cooling in (a) and (c) may be effected in a first heat exchange zone, the first liquid stream of (b) may be introduced into a stripping column, and a liquid stream enriched in hydrocarbons containing two or more carbon atoms and a residual vapor stream enriched in methane may be withdrawn from the column. In addition, the further-enriched hydrogen stream of (d) may be cooled and partially condensed in a second heat exchange zone to provide a two-phase stream, and the two-phase stream may be separated to yield a hydrogen vapor product stream and an additional hydrocarbon-enriched liquid stream.
The method may further comprise reducing the pressure of the additional hydrocarbon-enriched liquid stream to yield a reduced-pressure hydrocarbon-enriched liquid stream, warming the reduced-pressure hydrocarbon-enriched liquid stream in the second heat exchange zone to provide an additional two-phase stream, separating the additional two-phase stream to provide a vapor stream containing hydrocarbons and residual hydrogen and a liquid stream further enriched in hydrocarbons, and introducing the liquid stream further enriched in hydrocarbons into the top of the stripping column. The vapor stream containing hydrocarbons and residual hydrogen may be warmed in the first heat exchange zone to provide a portion of the cooling of the feed gas in (a).
The method may further comprise warming the hydrogen vapor product stream in the second heat exchange zone to provide by indirect heat exchange a portion of the cooling of the further-enriched hydrogen stream and further warming the hydrogen product stream in the first heat exchange zone to provide by indirect heat exchange a portion of the cooling of the feed gas in (a). The residual vapor stream may be warmed in the first heat exchange zone to provide by indirect heat exchange a portion of the cooling of the feed gas in (a). A portion of the feed gas stream may be cooled by indirect heat exchange with one or more hydrocarbon-rich liquid streams withdrawn from a lower part of the stripping column to provide a cooled feed stream and one or more vaporized hydrocarbon-rich streams, the one or more vaporized hydrocarbon-rich streams may be returned to the stripping column to provide boil-up therein, and the cooled feed stream may be combined with the partially condensed feed of (a).
The cold gas refrigerant generated in the closed-loop gas expander refrigeration cycle may provide cooling in the first and second heat exchange zones by the steps of
(1) providing a compressed refrigerant gas, dividing the compressed refrigerant gas into a first compressed refrigerant gas stream and a second compressed refrigerant gas stream, and work expanding the first compressed refrigerant gas stream to a first pressure to provided a first cooled work-expanded refrigerant gas stream;
(2) cooling the second compressed refrigerant gas stream in the first heat exchange zone to provide a cooled second compressed refrigerant gas stream;
(3) dividing the cooled second compressed refrigerant gas stream into a first portion and a second portion, work expanding the first portion to the first pressure to provide a second cooled work-expanded refrigerant gas stream, and further cooling the second portion in the first heat exchange zone to provide an intermediate cooled compressed refrigerant gas stream;
(4) warming the second cooled work-expanded refrigerant gas stream in the first heat exchange zone to provide a partially-warmed second work-expanded refrigerant gas stream and provide by indirect heat exchange a portion of the cooling of the feed stream in (a), and combining the partially-warmed second work-expanded refrigerant gas stream with the first cooled work-expanded refrigerant gas stream of (1) to provide a combined cooled work-expanded refrigerant gas stream;
(5) warming the combined cooled work-expanded refrigerant gas stream in the first heat exchange zone to provide by indirect heat exchange a portion of the cooling of the feed gas in (a), thereby providing a first warmed refrigerant gas stream;
(6) further cooling the intermediate cooled compressed refrigerant gas stream of (3) to provide a cold compressed refrigerant gas stream, reducing the pressure of the cold compressed refrigerant gas stream to a second pressure by either work expansion or Joule-Thomson expansion across a throttling valve, wherein the second pressure is lower than the first pressure, to provide a cold reduced-pressure refrigerant gas stream;
(7) warming the cold reduced-pressure refrigerant gas stream to provide by indirect heat exchange a portion of the cooling of the further-enriched hydrogen stream of (d) in the second heat exchange zone and a portion of the cooling of the feed gas of (a) in the first heat exchange zone, thereby providing a second warmed refrigerant gas stream; and
(8) compressing the first and second warmed refrigerant gas streams to provide the compressed refrigerant gas in (1).
The refrigerant gas may be selected from the group consisting of nitrogen, methane, a mixture of nitrogen and methane, and air.